The invention relates to a permanent magnet separating device. Problems still exist with the prior-art techniques of sorting and separating fine and very fine, magnetizable and slightly magnetizable granular material from finely ground particulate material (particle size 5 to 10 microns) having a dry, pasty or more or less fluid consistency, as well as with the prior-art techniques of sorting and separating magnetizable particles when the magnetizable particles are mixed with larger unmagnetic particles of a mixture, with the particles of such mixture being to some extent sintered or baked on to each other.
Prior-art magnetic separators, both automatic and non-automatic in the form of vertical filters, gratings, plates, rotating cylinders, etc., are provided with magnet arrangements so constructed that only the inhomogeneous magnetic stray or leakage flux is actually exploited to effect the separation of the magnetizable particles, with the major part of the magnetic force field passing through the interior of the system itself and not being made full use of. The leakage flux passing exteriorly of the magnetic circuitry in the prior-art arrangements and serving to effect the actual separation of magnetic particles from a mixture of magnetic and non-magnetic particles, will have a flux density of between about 300 and 1000 gausses, depending upon the particular construction. With some constructions it is even possible to achieve a flux density as high as 2000 gausses. When mainly the inhomogeneous leakage and stray flux of a magnetic circuit is employed to separate magnetic particles out of a mixture, it is not possible to separate out particles having particle sizes as small as mentioned above. Extensive laboratory testing has indicated that for the magnetic separation of such small magnetically attractable particles from the non-magnetic remainder of a mixture of fine particles, a flux density more on the order of 10,000 gausses is required, and in very difficult cases the necessary flux density to produce a satisfactory separation may even be as high as 30,000 gausses.
Attempts have already been made to improve the technique of magnetic sorting by building magnetic circuits which develop a highly directed, highly homogeneous magnetic field. One such construction makes use of two parallel magnetic cylindrical separator drums spaced apart from each other to form an adjustable separating zone or gap. In the interior of each of the two separator drums is located a stationary magnet oriented in direction towards the gap between the parallel drums so that magnetic flux will cross the gap in passing from one internal magnet to the other internal magnet. A return-flow path for magnetic flux is provided externally of the drums to form a magnetic circuit including the air gap between the drums, with the magnetic flux passing through the circuit thereupon passing through the air gap between the separator drums.
Another prior-art separator, in contrast to the one just described, comprised a single rotatably mounted cylindrical magnet drum provided at its circumferential periphery with axially extending bars of ferromagnetic material alternating with axially extending bars of non-magnetic material. The construction is in other respects the same as the one just described. One pole of a magnet located inside the drum is positioned in proximity to the inner periphery of the separator drum, while the opposite pole of the magnet is connected to a magnetic circuit branch extending from such opposite pole and ending at a distance from the separator drum with such an orientation as to form a radial air gap with the periphery of the drum. In order to increase the force of magnetic attraction, additional magnets can be connected in the magnetic circuit with their poles so oriented as to increase the flow of magnetic flux through the circuit, and advantageously being positioned at the portion of the magnetic circuit near the aforementioned gap formed with the separator drum. Between the magnet drum and the auxiliary pole just referred to, one prior-art construction has further included a solid cylindrical roller made from iron and magnetized by a further auxiliary pole extending from the basic magnetic circuit and partially including and magnetizing the solid cylindrical roller. With such a construction, the solid roller of iron contributed to the separation of the magnetic components by the magnet drum.
Using such improved constructions higher magnetic flux densities have been achieved, about 7,000 gauss. Stronger fields cannot be established because, on the one hand, the positioning of the magnetizing magnets within the interior of the hollow separator drum is limited both with respect to size and orientation by the limitation of space in the drum interior. Furthermore, the magnetizing magnet arrangements located inside the hollow separator drum are of course positioned with some clearance from the rotating inner periphery of the drum. The flux passing through the magnetic circuit must cross not only the work gap adjacent to the exterior drum periphery, but must also pass across this interiorly located gap. This results in the development of a significant stray flux which does not contribute to the effectiveness of the separating operation.